The present disclosure relates to a communication apparatus and a signal processing method. More particularly, the present invention relates to a communication apparatus and a signal processing method able to use the short-range communication conducted with respect to technology such as smart cards (i.e., IC cards), for example.
Recently, portable devices such as smart cards and mobile phones having short-range communication functions are being widely used. There is, for example, the FeliCa™ smart card developed by Sony. A short-range wireless communication standard called NFC (Near Field Communication) has also been developed by Sony and Philips.
In near field communication, a carrier frequency such as 13.56 MHz is used to communicate over a range from contact (i.e., 0 cm) to approximately 10 cm. Such communication will be summarized with reference to FIGS. 1A to 2B. At these distances, communication may be thought of as magnetic coupling between two transformers, with the transmit and receive antenna acting as coils.
FIG. 1A illustrates a process for transmitting data from a reader/writer 10 to a smart card or similar transponder 20, for example. FIG. 2A illustrates a process for transmitting data from the transponder 20 to the reader/writer 10.
The process for transmitting data from the reader/writer 10 to a smart card or similar transponder 20 will now be described with reference to FIGS. 1A and 1B. As shown in FIG. 1A, the reader/writer 10 modulates 212 kbps transmit information (the signal S1b) onto a 13.56 MHz carrier signal (the signal S1a) to produce a modulated signal (the signal S1c), which is then transmitted from a transmit amp to the transponder 20 via a coil. The transponder 20 receives an incoming (i.e., receive) signal (the signal S1d) via another coil.
FIG. 1B illustrates the signal waveforms of the carrier signal (signal S1a), the transmit information (signal S1b), the transmit signal (signal S1c), and the incoming (i.e., receive) signal (signal S1d). Herein, amplitude shift keying (ASK) is adopted as the modulation method.
The process for transmitting data from a smart card or similar transponder 20 to the reader/writer 10 will now be described with reference to FIGS. 2A and 2B. As shown in FIG. 2A, the reader/writer 10 transmits a 13.56 MHz carrier signal (the signal S2a) from a transmit amp to the transponder 20 via a coil. The transponder 20 modulates 212 kbps transmit information (the signal S2b) onto the carrier signal, and then transmits the generated transmit signal (the signal S2c) to the reader/writer 10. The reader/writer 10 receives an incoming (i.e., receive) signal (the signal S2d) via the coil.
FIG. 2B illustrates the signal waveforms of the carrier signal (signal S2a), the transmit information (signal S2b), the transmit signal (signal S2c), and the receive signal (signal S2d).
The reader/writer 10 and the transponder 20 shown in FIGS. 1A and 2A communicate over a range from contact (i.e., 0 cm) to approximately 10 cm. At these distances, communication may be thought of as magnetic coupling between two transformers, with the transmit and receive antenna acting as coils.
The characteristics of these transformers are such that each coil resonates with the carrier frequency at a high Q. By resonating near the carrier frequency, signals are amplified to enable transmission over greater distances. However, if the two resonating coils are in close proximity and mutually interfere, then the frequency characteristics of the transmission signal become like those shown in FIG. 3. As shown in FIG. 3, the resonant peak splits into two, with the carrier frequency lying in the valley between the two peaks.
FIG. 3 illustrates the correspondence between frequency and antenna level in two antennas made up of coils, for respective antenna distances between 0.5 mm to 100 mm. In other words, FIG. 3 illustrates the frequency characteristics of the transmission signal. For example, at antenna distances of 50 mm and 100 mm, the antenna level exhibits just one peak near the carrier frequency of 13.56 Mhz. At antenna distances of 30 mm, 6 mm, and 0.5 mm, the antenna level splits into two resonant peaks. Such splits are due to mutual interference of the two resonating coils when the antenna distance is decreased. As a result, the 13.56 MHz carrier frequency falls into the valley between the two peaks.
The above occurs because the resonant frequency of the two coils varies according to the inter-coil distance. The principle behind this variation will now be described with reference to FIGS. 4 and 5. FIG. 4 illustrates two coils a and b, which correspond to the respective coils in the reader/writer 10 and the transponder 20 shown in FIGS. 1A and 2A. The impedance Z0(s) to the right of the point P indicated by the arrow in FIG. 4 is given by the following Eq. 1.
                              Z                      O            ⁡                          (              s              )                                      =                              G                          (              s              )                                                          {                                                s                  2                                +                                  CL                  ⁡                                      (                                          k                      +                      1                                        )                                                              }                        +                          {                                                s                  2                                +                                  CL                  ⁡                                      (                                          k                      -                      1                                        )                                                              }                                                          Eq        .                                  ⁢        1            
In Eq. 1, k is a coupling coefficient, while G(s) is a third-order function of s (the resonant frequency). The resonant frequencies ω01 and ω02 are as follows, expressed as the roots of s computed from the above Eq. 1.
                                          ω                          O              ⁢                                                          ⁢              1                                =                                    1                              CL                                      ·                          1                                                1                  +                  k                                                                    ⁢                                  ⁢                              ω                          O              ⁢                                                          ⁢              2                                =                                    1                              CL                                      ·                          1                                                1                  -                  k                                                                                        Eq        .                                  ⁢        2            
Eq. 2 thus demonstrates that the resonant frequencies ω01 and ω02 differ according to the value of the coupling coefficient k. FIG. 5 illustrates how the resonant frequencies ω01 and ω02 vary according to the coupling coefficient k. FIG. 5 illustrates the correspondence between frequency and antenna level for values of the coupling coefficient k from 0.01 to 0.05.
Frequency characteristics near the carrier frequency (13.56 MHz) are influenced by the frequency characteristics of the detection signal (i.e., the signal reverted to the baseband). In other words, the frequency characteristics of the baseband signal match those of the carrier frequency near the DC level. FIG. 6 illustrates the frequency characteristics of a baseband signal obtained by decoding a carrier that has passed through a system having the characteristics shown in FIG. 5.
FIG. 6 illustrates baseband frequency characteristics, and shows the correspondence between the frequency and the relative (linear) level of the incoming signal strength for various values of the coupling coefficient k from 0.01 to 0.5. The level of the 13.56 MHz carrier frequency shown in FIG. 5 corresponds to the DC (i.e., 0 Hz frequency) level in FIG. 6. The average of the measured incoming signal levels (13.56+X) MHz and (13.56−X) MHz in FIG. 5 is thus equivalent to the relative level of the frequency X (MHz) shown in FIG. 6.
FIG. 6 compares the frequency characteristics over the frequency interval from 0.625 MHz to 1.25 MHz for different values of the coupling coefficient k.
When the coupling coefficient k is 0.2, the frequency characteristics over the frequency interval from 0.625 MHz to 1.25 MHz tend toward the upper range, as shown by the double broken line a. In other words, there is a tendency for the relative level of the incoming signal strength to increase as the frequency increases.
In contrast, when the coupling coefficient k is 0.1, the frequency characteristics over the frequency interval from 0.625 MHz to 1.25 MHz tend toward the lower range, as shown by the double broken line b. In other words, there is a tendency for the relative level of the incoming signal strength to decrease as the frequency increases.
If the communication distance is estimated according to matching points (i.e., peak frequencies) in FIGS. 3 and 5, then the frequency characteristics for the coupling coefficient k equal to 0.2 shown in FIG. 5 corresponds to an antenna distance of approximately 14 mm, while the frequency characteristics for the coupling coefficient k equal to 0.1 shown in FIG. 5 corresponds to an antenna distance of approximately 20 mm. This large variation in the incoming signal strength and resonant frequency for small differences in distance is characteristic of the present communication method.
When the coupling coefficient k is small (i.e., when the communication distance is large), the upper range attenuates sharply. However, when the coupling coefficient k increases (i.e., when the communication distance becomes smaller), the signal level peaks in the upper range.
However, such variation in the frequency characteristics has not posed a significant problem in the related art. This is because the transmission rates used in the systems of the related art are not particularly large. For example, in the FeliCa™ and NFC (Near Field Communication) standards, Manchester code at a transmission rate of 212 kbps is implemented. In other words, the frequency of the highest repeating waveform is 212 kHz.
FIG. 6 shows that when the communication distance separating the antennas is large, 212 kHz is approximately halved in level with respect to DC, and is flat at most distances. Consequently, the signal is large unaltered by the frequency characteristics of the channel (i.e., the transmission path), and the determination of 1s and 0s in the incoming signal is unhindered at the receiver.
However, if the transmission rate is increased, then the baseband signal spectrum widens by a factor of the rate increase, increasing the range of frequencies to be detected for the incoming signal. For this reason, the effects exerted by frequency characteristics of the channel increase, causing an increase (i.e., a worsening) in the data error rate.
A typical configuration of an incoming signal detector circuit in a communication apparatus of the related art, as well as the detection signals in such a detector circuit, will now be described with reference to FIGS. 7A and 7B. FIG. 7A illustrates the configuration of an incoming signal detector circuit in a communication apparatus of the related art. The configuration shown in FIG. 7A corresponds to the detector circuit 21 of the smart card or similar transponder 20 shown in FIG. 1, or to the detector circuit 11 of the reader/writer 10 shown in FIG. 2, for example. FIG. 7B illustrates signal waveforms at respective points along the detector circuit shown in FIG. 7A.
As shown in FIG. 7A, the detector circuit includes an amplifier 31, a wave detector 32, a high-pass filter (HPF) 33, and a comparator 34. An incoming signal is input via a coil that acts as an antenna, and exhibits an input waveform like that of the signal S3a shown in FIG. 7B.
This input waveform (signal S3a) is suitably amplified or attenuated in the amplifier 31 so as to exhibit sufficient amplitude. The amplifier 31 outputs a signal S3b like that shown in FIG. 7B. In some cases, the amplifier 31 herein may be realized by means of an attenuator or automatic gain controller (AGC), for example.
The output (signal S3b) of the amplifier 31 is input into the wave detector 32 and processed such that amplitude information regarding the amplitude signal is extracted therefrom. As a result, the wave detector 32 outputs a signal S3c like that shown in FIG. 7B.
The detection signal (signal S3c) of the wave detector 32 is then input into the high-pass filter (HPF) 33. The high-pass filter (HPF) 33 removes the DC component of the signal by setting the median potential of the waveform as the zero level, thereby generating a detection waveform from which the DC offset has been removed. The generated signal is the signal S3d shown in FIG. 7B.
The output of the high-pass filter (HPF) 33 (i.e., the detection waveform minus the DC offset (signal S3d)) is input into the comparator 34. The comparator 34 generates and outputs a binary (1/0) signal, using the zero level as the threshold value. In other words, the comparator 34 generates and outputs the signal S3e shown in FIG. 7B as the incoming (i.e., receive) information waveform.
Incoming signal detector circuits for near field communication in the related art are configured as shown in FIG. 7A, and as a result of the signal processing produced by such a configuration, output received information in the form a binary signal (signal S3e) generated from an incoming signal (signal S3a).
The above configuration enables processing without problems for low transmission rates of approximately 212 kbps. However, if the transmission rate is increased, the signal might be significantly altered by the frequency characteristics of the channel (i.e., the transmission path), and the ability to accurately determine 1s and 0s from the detection waveform by removing the DC offset might be compromised.
In light of the foregoing problems, it is desirable to provide a communication apparatus and a signal processing method able to reduce the error rate and accurately receive data, even at fast transmission rates.